The present invention relates to a process for preparing a catalyst for removing sulfur and nitrogen and converting the 370.degree. C.+ fraction of vacuum gas oil, used as feedstock, to middle distillates. More especially, the invention relates to a catalyst for MHC of heavy vacuum gas oils, and the operative working conditions for obtaining maximum efficiency thereof.
In general, mild hydrocracking (MHC) consists of bringing into contact a charge containing sulfur, nitrogen, aromatics, olefins, saturates and a high concentration of the fraction which distills above 370.degree. C., with a catalyst in the presence of hydrogen and under specified conditions of pressure, temperature and volumetric space velocity for the purpose of converting the sulfur compounds to H.sub.2 S, the nitrogen compounds to NH.sub.3, hydrogenating the aromatic compounds and converting the 370.degree. C.+ fraction to middle distillates which distill in the range between 250.degree. and 370.degree. C.
A typical catalyst for hydrotreatment, composed of hydrogenating metals of Group VIB and Group VIII of the Periodic Table of the elements, supported on gamma alumina, can be used in mild hydrocracking (MHC), but the life of the catalyst is, however, low, generally fluctuating between 12 and 18 months, and this results in replacement of the catalyst and an increase in the cost involved in the operation.
For this reason, various procedures have been described which aim to increase the activity and selectivity of catalysts in respect of hydrodesulfurization (HDS) and in respect of hydrodenitrogenation (HDN) of different cuts of petroleum origin, but there is nevertheless little information in the prior art in regard to the analysis, preparation and use of catalysts of hydrotreatment for converting the 370.degree. C.+ fraction of vacuum gas oils to middle distillates, and increasing their life in the process. A catalyst of hydrotreatment composed of metals of Group VIB and Group VIII of the Periodic Table has been claimed in U.S. Pat. Nos. 3,232,887 and 3,287,280. The catalyst possesses high activity for HDS and HDN of petroleum charges, but is not applied for mild hydrocracking. In particular, U.S. Pat. No. 3,287,280 describes a method and the impregnating solutions for preparing a molybdenum-nickel catalyst which also contains phosphorus in its composition, but the investigators did not succeed in determining in their studies the optimal ratio of metals to phosphorus required in the final catalyst for the latter to be active in hydroconversion of the heavy fractions of the charges of petroleum origin to middle distillates.
In U.S. Pat. No. 3,755,150, Mickelson claims a superior catalyst for hydrodesulfurization of the said charges, which catalyst is prepared using an impregnating solution which exhibits a P/MoO.sub.3 ratio between 0.1 and 0.25, and has a pH between 1 and 2. Although the investigator establishes that the use of high ratios of phosphorus to molybdenum is critical for obtaining an excellent catalyst in HDS and HDN, no analysis was made of the surface concentration of metals, which is critical. We have discovered that a P/(Mo+Ni) ratio of between 0.7 and 1.1 confers stability and high activity on the catalyst.
In U.S. Pat. No. 4,396,500, Simpson describes a process and a catalyst for hydrotreatment in which the catalyst consists of molybdenum, nickel and phosphorus deposited on a support of the gamma alumina type which has been precalcined at a temperature of 1375.degree. F. The composition of the catalyst contains between 12 and 30% by weight of molybdenum, between 2 and 6% by weight of NiO and between 2 and 6% by weight of phosphorus, calculated as P. It is clear that in the composition of the catalyst, phosphorus is in a stoichiometric ratio with respect to the metal of Group VIII of the Periodic Table, and the said composition does not apply in our case, since only if the phosphorus is in stoichiometric proportions with respect to the Group VIB and Group VIII metals it is possible to obtain maximum efficiency in the composition and in the catalytic activity of the catalyst.
None of the processes of the prior art, for preparing catalysts of hydrotreatment, is there mentioned a process for preparing a catalyst which is active and stable in the hydroconversion of the 370.degree. C.+ fraction of vacuum gas oils to middle distillates, and which also shows excellent activity in HDS and HDN. The high concentration of metals on the surface of the catalyst and the high capacity it possesses for being sulfurized are the features which account for the significant advantages of this new catalyst compared with the catalysts of the prior art. Furthermore, the new catalyst produces less coke than the previous catalysts, which endows it with an additional advantage since it can operate for a longer period (26 months as against 14 months).